Nuclear Component

Pregnancy Miracle

The Pregnancy Miracle

Get Instant Access

According to Gaulden (9), hypoxia is responsible for a reduction in metabolic activity as well as for a change in internal pH both of which are likely to affect organization and integrity of the meiotic metaphase spindle. This is supported by data from pre-antral follicle culture indicating that in vitro maturation at 5% oxygen tension (instead of 20%) resulted in a significant reduction of gametes finishing nuclear maturation (10), e.g., characterized by a complete spindle absence. More interestingly, the rate of unaligned chromosomes increased dramatically from 13% in the 20% oxygen group to 35% in the low oxygen cohort. Similarly, Van Blerkom et al. (11) reported that 92% of the oocytes exhibiting chromosome displacement or abnormal chromosomal alignment originated from follicles with dissolved oxygen contents of less than 3%.

Considering the importance of the follicular and in vitro milieu and its close relationship to the health of the gamete, it is not surprising that up to 38% of analyzed MII oocytes lack a spindle apparatus as shown using a polarized light microscope (12). Though detectability improved with increasing PolScope experience (Table 1), one characteristic remained consistent, namely a reduction in developmental competence in spindle negative mature oocytes as assessed by fertilization rate (13,15,16). Even in spindle positive gametes, grading in terms of fertilizability is suggested (16) with those oocytes of worst quality showing a spindle deviation of more than 90 degrees from the first polar body.

However, absence of a birefringent spindle does not predict fertilization failure and developmental arrest. In fact, it has been found that human oocytes with a polar body but without birefringent spindle may still be at telophase I or prometaphase I stage (18). Thus, precocious intracytoplasmic sperm injection (ICSI) in human prometaphase I oocytes with unaligned chromosomes may be one reason why oocytes without a birefringent metaphase II spindle have a significantly worse prognosis.

Knowing the actual position of the birefringent spindle during ICSI can improve embryo quality (14). If no spindle at all has been detected, the probability of a good quality embryo decreases dramatically (13,15), though this suspected correlation is still subject to controversial discussion (16,17).

Table 1 Visualization of Metaphase II Spindle by Means of Polscope and its Influence on Fertilization Rate

Spindle in

Fertilization rate

proximity to

Authors

Spindle positive

Pb1

Spindle

No spindle

Wang et al. (12)

327/533 (61.4)

61 (18.7)

202 (61.8)a

91 (44.2)a

Wang et al. (13)

1266/1544 (82.0)

Nd

879 (69.4)b

175 (62.9)b

Cooke et al. (14)

115/124 (92.7)

35 (30.4)

81 (70.4)

Nd

Moon et al. (15)

523/626 (83.6)

252 (48.2)

444 (84.9)c

78 (75.7)c

Rienzi et al. (16)

484/532 (91.0)

254 (52.5)

362 (74.8)d

16 (33.3)d

Cohen et al. (17)

585/770 (76.0)

Nd

413 (70.6)e

115 (62.2)

Abbreviations: Nd: no data available; Pb: first polar body; Values in parentheses are percentages.

Source: From Ref. 18a.

Abbreviations: Nd: no data available; Pb: first polar body; Values in parentheses are percentages.

Source: From Ref. 18a.

The only paper correlating spindle detection with further preimplanta-tion development to the blastocyst stage (13) reports increased rates of blastocyst development by day five arising from spindle-positive oocytes (51.1%) compared with the spindle-negative counterparts (30.3%), thus supporting the hypothesis that spindle detection may be used as indicator of the oocyte's capacity to form a viable, chromosomally balanced embryo.

In addition, oocytes rather tend to show a visible spindle apparatus if postovulatory age exceeds 38 hours (17), making spindle imaging a new marker for optimal timing of the ICSI procedure and thus increasing the chance to generate viable embryos.

First polar body morphology takes the same line, since the most notable characteristic of postovulatory aging is the spontaneous division or fragmentation of the first polar body (19). Bearing this in mind, it is not surprising that a close correlation between the first polar body appearance and the further fate of the oocyte was observed (20-23). In detail, heavily fragmented first polar bodies were negative predictors of embryo quality, blastocyst formation rate as well as rates of implantation and clinical pregnancy. Apparently this benefit is somewhat reduced with increasing time span between ovulation induction and injection, since a retrospective study applying a different schedule could not find any relationship between constitution of the first polar body and subsequent ICSI outcome (24).

In contrast to postovulatory age, chromosomal status of the oocyte is not reflected by the morphology of the first polar body as suggested from data of polar body biopsy. Regardless of the grade of the first polar body, more than two-thirds of the oocytes were found to be aneuploid (25), but, unfortunately, the most interesting grade consisting of large polar bodies was not analyzed in this highly selected patient cohort.

It has been summarized that MII oocytes of good morphology should be of regular size and show a clear, moderately granulate cytoplasm, a small perivitelline space, and a colorless zona pellucida (2). As a precaution, eggs with an observed deviation in size should not be kept in culture since, e.g., giant oocytes will mostly result in trigynic triploidy (26,27). On the other hand, any reduction in diameter might reflect a certain cytoplasmic loss during manipulation of the oocyte (28).

Was this article helpful?

0 0
Get Pregnant - Cure Infertility Naturally

Get Pregnant - Cure Infertility Naturally

Far too many people struggle to fall pregnant and conceive a child naturally. This book looks at the reasons for infertility and how using a natural, holistic approach can greatly improve your chances of conceiving a child of your own without surgery and without drugs!

Get My Free Ebook


Post a comment